The present invention relates to an optical cantilever for observing the shape of a sample by utilizing an atomic force occurring between substances and measuring optical characteristics in a very small area, and to a method of fabricating such an optical cantilever.
At present, using a scanning type near-field microscope (hereinafter, abbreviated as an SNOM), optical characteristics or the shape of a sample are measured by placing a probe comprising an optical medium and having a front end that is sharpened, proximate to the sample to be measured by a distance the size of the wavelength of light or smaller. There has been proposed an apparatus in which a front end of an optical fiber probe having a linear shape is held vertically relative to a sample and is vibrated horizontally relative to a surface of the sample, so that detection of a change in an amplitude of vibration caused by shear force between the surface of the sample and the front end of the probe, is carried out by irradiating a laser beam to the front end of the probe and detecting a change in a shadow thereof and an interval between the front end of the probe and the surface of the sample is maintained constant by moving the sample by a fine movement mechanism such that the amplitude becomes constant to thereby detect a shape of the surface from an intensity of a signal inputted to the fine movement mechanism and measure optical characteristics of the sample.
Further, there has been proposed a scanning type near-field atomic force microscope using an optical fiber probe formed in a shape of a hook as a cantilever of an atomic force microscope (hereinafter, abbreviated as AFM) to thereby carry out AFM operation and, at the same time, irradiating laser beam from a front end of the optical fiber probe to a sample to thereby detect a shape of a surface of the sample and measure optical characteristics thereof (Japanese Patent Laid-Open No.174542/1995). FIG. 11 is a constitution view showing an optical fiber probe of a conventional example.
According to the optical fiber probe, there is used an optical fiber 501 and a surrounding of the optical fiber 501 is covered with a metal film coating 502. Further, a stylus portion 503 is sharpened and a front end of the stylus portion 503 is provided with an aperture 504.
Meanwhile, according to AFM which is utilized as means for observing a shape of a very small area, there is widely utilized a micro cantilever of silicon or silicon nitride fabricated by a silicon process. The micro cantilever used in AFM is provided with high resonance frequency, excellent mass production performance and small dispersion of shape and therefore, the micro cantilever is characterized in that mechanical properties such as spring constant, resonance frequency and the like are uniform.
In observation by SNOM and AFM, in order to carry out scanning control at high speed, the resonance frequency of the optical fiber probe needs to be high, meanwhile, in order to measure a soft sample such as a sample of an organism which is one of observation objects of SNOM without damaging the sample, spring constant of the optical fiber probe must be reduced. However, according to the optical fiber probe, the optical fiber per se is used as a spring material of the cantilever and therefore, it is difficult to simultaneously realize to increase the resonance frequency and reduce the spring constant and there poses a problem that it is difficult to observe a soft sample at high speed without damaging the sample.
Further, the optical fiber probe is fabricated by manual operation in many steps with the optical fiber as a material and there poses a problem that the mass production performance is low and it is difficult to make uniform a shape thereof such as a diameter of the front end or an angle of the front end.
Hence, the invention has been carried out in view of the above-described and it is an object thereof to provide an optical cantilever which is an optical cantilever for SNOM irradiating and/or detecting light to and from a very small aperture, excellent in mass production performance and uniformity and capable of observing even a soft sample at high speed without damaging the sample and a method of fabricating thereof.
In order to achieve the above-described object, according to an aspect of the invention, there is provided an optical cantilever comprising a base portion, a cantilever portion extending from the base portion, a dielectric member formed to penetrate the cantilever portion and project above the cantilever on a side opposed to the base portion and having a sharpened front end, a light shielding film for covering a surrounding of the sharpened dielectric member, and a very small aperture formed at the sharpened front end of the dielectric member.
Therefore, according to the optical cantilever of this aspect of the invention, light can be emitted from the very small aperture to the sample by making light incident from the side opposed to the sharpened front end of the dielectric member. Further, light can be detected by the very small aperture. Further, according to the optical cantilever of this aspect of the invention, resonance frequency and spring constant thereof can be adjusted by dimensions of the cantilever portion and accordingly, high resonance frequency and small spring constant can be provided and a soft sample can be observed at high speed without damaging the sample. Further, the optical cantilever according to this aspect of the invention is provided with a shape similar to that of a cantilever of an AFM having a base portion and a cantilever portion extending from the base portion and accordingly, accumulated technology of the AFM can effectively be applied and it is simple to deal therewith.
Further, according to another aspect of invention, there is provided a method of fabricating an optical cantilever characterized in including a step of forming a hole to penetrate a cantilever portion, a step of depositing a dielectric member in the hole, a step of sharpening the dielectric member, and a step of depositing a light shielding film on the sharpened dielectric member and forming a very small aperture.
Therefore, the optical cantilever of the invention can be fabricated by a silicon process, the conventional semiconductor fabricating technology and the technology of fabricating the cantilever of AFM can effectively be applied, which is excellent in mass production performance and uniformity.
Further, according to the optical cantilever of the invention, by providing a step of removing the dielectric member in the method of fabricating thereof, the dielectric portion is constituted by a cavity.
Therefore, absorption of light in the atmosphere or in vacuum at the dielectric member can be disregarded and therefore, there can be widely selected a wavelength range which can be used by incident light.
Further, according to the optical cantilever of the invention, the face on the side opposed to the sharpened front end of the dielectric member is constituted by a projected shape. Thereby, light can be focused to the very small aperture portion and accordingly, the intensity of emitted light can be increased.
Further, according to the optical cantilever of the invention, the face on the side opposed to the sharpened front end of the dielectric member is formed at a position not projected from the surface of the cantilever portion on the side the same as the side of the base portion. Thereby, introduced light reflected by the dielectric member on the side opposed to the sharpened front end of the dielectric member is not leaked and the S/N ratio in measurement by SNOM can be increased.
Further, according to the optical cantilever of the invention, in a method of fabricating the optical cantilever, there is provided a step of adjusting a thickness of the cantilever by thinning the cantilever portion from a direction of depositing dielectric member, thereby, the height of the dielectric member having the sharpened front end can be increased and observation of a sample having a large stepped difference and observation in a solution can easily be carried out.
Further, the optical cantilever of the invention is constituted by an optical cantilever characterized in having a cantilever for an optical lever integral with the cantilever portion on the side opposed to the sharpened dielectric member relative to a fulcrum of the cantilever portion. Therefore, by irradiating laser beam of optical lever to the cantilever for an optical lever, light for observing the optical characteristics of the sample and light for measuring displacement of the cantilever can be separated from each other and accordingly, the S/N ratio in observation by SNOM can be increased. Further, by making the length of the cantilever for optical lever larger than the length of the cantilever portion, the sensitivity in measuring displacement of the cantilever can be increased.